A) Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a vertical alignment type liquid crystal display.
B) Description of the Related Art
A so-called vertical alignment type liquid crystal display (LCD) has two transparent substrates and a liquid crystal layer sandwiched between the substrates, and arranges liquid crystal molecules perpendicular to boundary planes between the liquid crystal layer and substrates or slightly inclined from the plane perpendicular to the boundary planes. Retardation of this liquid layer is 0 or almost 0 when viewed from the front side in the state that voltage is not applied. Therefore, if two polarizers are disposed in a cross nicol arrangement on the outer sides of the liquid crystal cell, a normally black type display having good black display quality can be manufactures because of the quenching performance of the two polarizers cross-nicol disposed.
As an example of a vertical alignment type LCD having good viewing angle characteristics, a multi-domain vertical alignment type LCD is known which controls liquid crystal molecule alignment to have a plurality of alignment directions in one pixel.
Multi-domain is realized by generating an oblique electric field between upper and lower substrates by forming slits in electrodes disposed on the inner surface of the substrate (e.g., refer to Japanese Patent Gazette No. 2507122) or by forming projections on the substrate plane (e.g., refer to Japanese Patent Gazette No. 2947350).
These methods of realizing multi-domain have disadvantages of a lowered aperture ratio and a lowered optical transmittance of LCD because a slit or projection is disposed in a pixel, although it is unnecessary to positively conduct an alignment process such as a rubbing process for substrate surfaces.
As an example of a vertical alignment type LCD avoiding a lowered aperture ratio and suppressing a reduction in an optical transmittance, a mono-domain pretilt vertical alignment type LCD is known which is provided with a uniform pretilt angle by positively conducting a uniform alignment process because LCD has an electrode structure similar to a conventional TN-LCD and its substrate surface is not irregular.
FIG. 7 is a schematic broken perspective view of a mono-domain pretilt vertical alignment type LCD.
The mono-domain pretilt vertical alignment type LCD is constituted of a pair of substrates (upper substrate 31 and lower substrate 32) and a liquid crystal layer 39 sandwiched between the substrates. The upper substrate 31 and lower substrate 32 are constituted of upper and lower transparent substrates 33 and 34 of, e.g., transparent parallel glass substrates, upper and lower transparent electrodes 35 and 36 formed on opposing planes of the upper and lower transparent substrates 33 and 34, made of transparent conductive material such as indium tin oxide (ITO) and having predetermined patterns, and upper and lower vertical alignment films 37 and 38 formed covering the upper and lower transparent electrodes 35 and 36.
The paired substrates (upper substrate 31 and lower substrate 32) are disposed generally in parallel making the vertical alignment films 37 and 38 face each other, and the liquid crystal layer 39 is sandwiched between the vertical alignment films 37 and 38. A voltage application means 43 is connected across the transparent electrodes 35 and 36 and can apply a desired voltage to the liquid crystal layer 39 between the transparent electrodes 35 and 36. FIG. 7 shows the state of the liquid crystal layer 39 when voltage is not applied across the transparent electrodes 35 and 36.
The upper alignment film 37, lower alignment film 38, or both the upper and lower alignment films 37 and 38 are subjected to a uniform alignment process (providing a uniform pretilt angle) to form a non-defective mono-domain LCD.
An alignment method includes (i) a method of forming a substrate having an anisotropic surface by oblique vapor deposition of SiO2 inorganic metal oxide or by in-line sputtering (e.g., refer to JP-A-HEI-11-160707), a method of forming thereafter a surface active agent film on the anisotropic substrate surface to use it as a polarizer film (e.g., refer to JP-A-HEI-11-160706), (ii) an optical alignment method of irradiating ultraviolet rays to a photosensitive vertical polarizer along an oblique direction relative to the film surface (e.g., refer to Japanese Patent Gazette No. 2872628), (iii) a method of rubbing a vertical polarizer film having a proper surface free energy under proper conditions, and other methods.
Upper and lower polarizers 41 and 42 are disposed in a cross nicol arrangement on outer sides of the paired substrates (upper substrate 31 and lower substrate 32) generally in parallel. Directions of transmission axes of the polarizers 41 and 42 are indicated by arrows. Each of the polarizers 41 and 42 transmits only light polarized along the transmission axis direction.
In the state that no voltage is applied, light incident upward along the display normal is polarized by the lower polarizer 42 along a direction parallel to the arrow direction, transmits through the liquid crystal layer 39 and is intercepted by the upper polarizer 41. Therefore, the vertical alignment type LCD displays “black”.
In the state that voltage is applied, the alignment state of liquid molecules 39a changes from that when no voltage is applied. Light incident from the lower polarizer 42 side has optical components along the transmission axis direction of the upper polarizer 41 and transmits through the upper polarizer 41. Therefore, LCD displays “white”.
As shown in FIG. 7, X- and Y-directions (arrow direction is a positive direction) are defined which are perpendicular in the in-plane direction of the upper substrate 31 and lower substrate 32. A Z-direction is also defined which extends along a direction perpendicular to the upper substrate 31 and lower substrate 32 and has a positive direction from the lower substrate 32 toward the upper substrate 31, to thereby incorporate the right-handed coordinate system. Angle coordinates in the substrate in-plane counter-clockwise direction (rotation direction toward the negative X-direction) are defined having the positive Y-direction as a 0° azimuth. With these angel coordinates, the negative X-direction is a 90° azimuth, the negative Y-direction is a 180° azimuth, and the positive X-direction is a 270° azimuth.
The azimuth (azimuth indicated by the arrow) of the transmission axis of the upper polarizer 41 is a 45°-225° azimuth, and the azimuth of the transmission axis of the lower polarizer 42 is a 135°-315° azimuth.
The vertical alignment type LCD is associated with a problem of a lowered contrast due to optical through when viewing at a deep polar angle relative to the substrate normal. Deterioration of viewing angle characteristics caused by optical through is considerable particularly in the state that no voltage is applied. It can be considered that there are two factors of forming optical through: exhibition of birefringence effects caused by an increase in a retardation of the liquid crystal; and viewing angle dependency of the polarizer.
Optical through by the viewing angle dependency of the polarizer occurs in the following manner. In the state that the polarizers are disposed in a cross nicol arrangement outside the upper and lower substrates, the apparent layout of the upper and lower polarizers shifts from the cross nicol state as the observation polar angle is made deeper along the direction other than the transmission axis or absorption axis of the polarizers. In an extreme case, a perfect parallel nicol state appears when viewing along the substrate in-plane direction (observation polar angle=90°). Namely, as the observation polar angle is made deep from the normal direction, the polarizer cross nicol state is diminished so that optical through occurs.
Optical through by an increase in a retardation can be improved by using a viewing angle compensation plate which, for example, if the liquid crystal layer has positive uniaxial optical anisotropy, is made of transparent medium having a negative optical anisotropy canceling out the positive optical anisotropy.
FIG. 8 is a schematic broken perspective view of a mono-domain pretilt vertical alignment type LCD having a viewing angle compensation plate.
This LCD differs from LCD shown in FIG. 7 in that a viewing angle compensation plate 45 is disposed between an upper transparent substrate 33 and an upper polarizer 41. The viewing angle compensation plate 45 may be inserted between one substrate and the polarizer as shown in FIG. 8, or may be inserted between both the polarizes and substrates.
FIG. 9 shows an observation polar angle dependency of an optical transmittance when a viewing angle compensation plate is used (vertical alignment type LCD shown in FIG. 8) and when a viewing angle compensation plate is not used (vertical alignment type LCD shown in FIG. 7).
The observation polar angle dependency was shown at the 0°-180° azimuth (right-left azimuth) in the no voltage application state under the condition of Rth≈Δnd−140 nm, where Rth is retardation of the viewing angle compensation plate and Δnd (Δn: liquid crystal material birefringence, d: thickness of liquid crystal layer 39) is a retardation of the liquid crystal layer 39.
The abscissa represents an observation angle (poplar angle) in the unit of “° (degree)”. This graph shows an inclination angle (observation angle, polar angle) from the positive Z-direction toward the positive Y-direction (0° azimuth) or toward the negative Y-direction (180° azimuth). An inclination angle from the positive Z-direction toward the positive Y-direction (0° azimuth) was indicated by a positive value, and an inclination angle from the positive Z-direction toward the negative Y-direction (180° azimuth) was indicated by a negative value. An absolute value of the negative observation angle is equal to an inclination angle from the positive Z-direction toward the negative Y-direction (180° azimuth).
The ordinate represents an optical transmittance at each observation angle in the unit of “%”.
A curve a shows the relation between an observation angle and an optical transmittance of a vertical alignment type LCD (vertical alignment type LCD shown in FIG. 7) not using a viewing angle compensation plate, and a curve b shows the relation between an observation angle and an optical transmittance of a vertical alignment type LCD (vertical alignment type LCD shown in FIG. 8) using a viewing angle compensation plate.
An optical transmittance (curve a) of LCD not using the viewing angle compensation plate is near 0 up to a polar angle of about 20°, gradually increases from the polar angle of about 20°, is 3% or larger at a polar angle of 60°.
An optical transmittance (curve b) of LCD using the viewing angle compensation plate is smaller than that (curve a) of LCD not using the viewing angle compensation plate, particularly at a polar angle of about 20° or larger, and is a half or smaller at a polar angle of 60°. As seen from this graph, by using the viewing angle compensation plate, optical through can be suppressed and good display quality can be realized particularly at a deep observation angle.
However, as seen from the curve b, optical through cannot be perfectly resolved even by the vertical alignment type LCD using the viewing angle compensation plate. This is because there remains optical through due to the viewing angle dependency of the polarizer.
There is a proposal of LCD capable of preventing optical through to be caused by both the retardation of a liquid crystal layer and the viewing angle dependency of a polarizer (e.g., refer to JP-A-HEI-11-258605).
FIGS. 10A to 10E are schematic broken perspective views of LCDs.
Reference is made to FIG. 10A. LCD shown in FIG. 10A differs from LCD shown in FIG. 7 in that a C plate 46 is additionally disposed between an upper substrate 31 and an upper polarizer 41 and an A plate 47 is additionally disposed between a lower substrate 32 and a lower polarizer 42.
Mutually perpendicular X-axis and Y-axis are defined in the in-plane direction of an optical film (phase difference plate) and a Z-axis is defined in a thickness direction. Refractive indices in the X-, Y- and Z-axes are represented by nx, ny and nz, respectively. The A plate has a refractive index distribution of nx>ny=nz, whereas the C plate has a refractive index distribution of nx≈ny>nz.
The A plate 47 is an optical film (phase difference plate) having a positive uniaxial optical anisotropy and an optical axis in the in-plane, and the C plate 46 is an optical film (phase difference plate) having a negative, almost uniaxial optical anisotropy and an optical axis in the thickness direction.
By using the A plate 47 and C plate 46, it becomes possible to prevent optical through to be caused both by the retardation of the liquid crystal layer and by the polarizer. This is because the C plate 46 (negative uniaxial optical anisotropy) has a function of canceling (compensating) the retardation (positive uniaxial optical anisotropy) of the liquid crystal layer during oblique observation, and the A plate 47 when used with the C plate 46 can realize an optical function of resolving the viewing angle dependency of the polarizer. An in-plane retardation Re of an optical film (phase difference plate) is defined by Re=(nx−ny)×d where d is a thickness of the optical film, and a thickness direction retardation Rth is defined by Rth=[{(nx+ny)/2}−nz]×d.
As shown in FIG. 10C, the A plate and C plate stacked together may be disposed on the upper surface of the upper substrate or on the lower surface of the lower substrate. In this case, advantages similar to those of FIG. 10A can be obtained even if the C plate is disposed on the plane near the cell and the A plate is disposed on the plane near the polarizer.
Reference is made to FIG. 10B. LCD shown in FIG. 10B differs from LCD shown in FIG. 7 in that a biaxial film 48 is additionally disposed between an upper substrate 31 and an upper polarizer 41.
The biaxial film 48 is an optical film having a negative biaxial optical anisotropy and collecting the functions of the A plate and C plate in one optical film. Namely, a negative biaxial film is an optical film defined by nx>ny>nz.
By using the biaxial film 48, advantages similar to those obtained when the A plate 47 and C plate 46 are used can be obtained.
FIG. 10B shows the structure of FIG. 10C reducing the number of optical films, and this structure can provide the optical characteristics generally equivalent to those of FIG. 10C).
As shown in FIGS. 10D and 10E, even if the A plate used in FIGS. 10A and 10C is replaced with the biaxial film, almost similar advantages can be obtained.
When the mono-domain pretilt vertical alignment type LCD shown in FIG. 7 is observed at deep observation polar angles, it is recognized that there appears the phenomenon that luminances (optical transmittances) are different between the 0° azimuth and the 180° azimuth, in the state that no voltage is applied or in the state that a voltage near a threshold voltage (corresponding to no selection of voltage in simple matrix drive) is applied to LCD for low luminance emission. For example, in a simple matrix drive display of a segment display type, different luminances (optical transmittances) appear depending on an observation azimuth, when a display area is in a non-selection state (off segment). In the state that no voltage is applied, it can be seen from FIG. 9 that optical transmittances at a polar angle of, e.g., 60° at the 0° azimuth and the 180° azimuth are definitely different.
The present inventor simulated the optical transmittance of a mono-domain pretilt vertical alignment type LCD under the no voltage application condition and the voltage near threshold voltage application condition, by using a pretilt angle as a parameter. A simulation target LCD had the structure that a C plate as the viewing angle compensation plate is inserted between the lower substrate and lower polarizer of the mono-domain pretilt vertical alignment LCD shown in FIG. 7. The observation azimuth used was the 0° azimuth and 180° azimuth described with reference to FIG. 7, and the observation polar angle was set to 50° (an angle inclined from the substrate normal direction toward the substrate in-plane by 50°).
Simulation was conducted by using an LCD simulator LCD master 6 manufactured by Thing Tech Co. Ltd. When other simulations described in this specification were conducted, this simulator was used.
The alignment process direction (rubbing direction) of the lower substrate of the simulation target LCD was set to the 270° azimuth, and that of the upper substrate was set to the 90° azimuth, to realize antiparallel alignment between the upper and lower substrates. The liquid crystal layer is made of liquid crystal material having a negative dielectric constant anisotropy (Δ∈<0), specifically Δ∈=−5.1 to set a retardation Δnd to about 0.36 μm. Chiral material was not added. SHC125U manufactured by Polatechno Co., Ltd was used for the upper and lower polarizers. The azimuths of transmission axes of the upper and lower polarizers were set to a 45°-225° azimuth and a 135°-315° azimuth, respectively. The C plate was made of norbornene resin and has a thickness retardation Rth of 220 nm.
FIG. 1 is a graph showing simulation results.
The abscissa of the graph represents a pretilt angle in the unit of “° (degree)” and the ordinate represents an optical transmittance in the unit of “%”.
A curve c1 shows the relation between a pretilt angle and an optical transmission when observing from the 0° azimuth in a no voltage application state. A curve c2 shows the relation when observing from the 180° azimuth in a no voltage application state. Curves d1 and d2 show the relations thereof when observing from the 0° azimuth and 180° azimuth, respectively, in a voltage near threshold voltage application state.
Reference is made to the curves c1 and c2. At a pretilt angle of 90°, there is no difference between optical transmittances when observing from the 0° azimuth and from the 180° azimuth. However, as the pretilt angle is made smaller, the optical transmittance linearly increases at the 0° azimuth, whereas the optical transmittance linearly decreases at the 180° azimuth. Therefore, the smaller the pretilt angle is, the larger a difference of optical transmittances between the 0° azimuth and 180° azimuth is.
Reference is made to the curves d1 and d2. Also in the voltage near threshold voltage application state, at a pretilt angle of 90°, optical transmittances are the same when observing from the 0° azimuth and from the 180° azimuth. This optical transmittance is equal to that when observing from the 0° azimuth and from the 180° azimuth in the no voltage application state.
However, as the pretilt angle is made smaller, the optical transmittance linearly increases at the 0° azimuth, whereas the optical transmittance linearly decreases at the 180° azimuth. An increase rate and a decrease rate are larger than those in the no voltage application state. Therefore, the more the polar angle azimuth is inclined, the larger a difference of optical transmittances becomes between the 0° azimuth and 180° azimuth, and the difference is large than that in the no voltage application state.
The simulation results coincide with the external observation of LCD.
In the graph shown in FIG. 1, the reason why the optical transmittance when observing at the 0° azimuth is larger than that when observing at the 180° azimuth is the uniaxial pretilt alignment and that the viewing angle compensation plates are asymmetrically disposed on the upper and lower sides of the liquid crystal cell. If the viewing angle compensation plates having quite the same characteristics are disposed on the upper and lower sides of the liquid crystal cell, it is possible to obtain the same optical transmittance both at the 0° and 180° azimuths, independently from the pretilt angle in the liquid crystal layer. However, the number of optical films used increases, resulting in disadvantages of cost. The transmittance at the 180° azimuth can be reversed to that at the 0° azimuth by changing the transmission axis azimuths of the upper and lower polarizers or moving the C plate from the lower side of the cell to the upper side of the cell.
Then, the inventor studied the relation between an in-plane direction retardation of a biaxial film and a minimum optical transmittance in the no voltage application state, by using LCD having the biaxial film as a substitute for the C plate of the above-described simulation target, i.e., by using a new simulation target having as a viewing angle compensation plate a biaxial film inserted between a lower substrate and a lower polarizer in the mono-domain pretilt vertical alignment type LCD shown in FIG. 7.
LCD used as the new simulation target is different from the above-described simulation target in that the C plate is replaced with a biaxial film and a retardation of the liquid crystal layer is set to Δnd of about 0.38 μm. A thickness direction retardation Rth of the biaxial film was set to 250 nm and an in-plane delay phase axis was set to a 135°-315° azimuth.
Similar to the above-described simulation, an observation azimuth was set to the 0° azimuth and 180° azimuth and an observation polar angle was set to 50°.
FIG. 2 is a graph showing the simulation results.
The abscissa of the graph represents an in-plane retardation Re of the biaxial film in the unit of “nm” and the ordinate represents an optical transmittance in the unit of “%”.
A curve e1 indicates the relation between the in-plane retardation Re and the minimum optical transmittance when observing along the 0° azimuth at a pretilt angle of 90°. A curve e2 indicates the relation when observing along the 180° azimuth at a pretilt angle of 90°. The curves e1 and e2 are the same and drawn superposed one upon the other.
A curve f1 and a curve f2 indicate the relations when observing along the 0° azimuth and 180° azimuth at a pretilt angle of 89°.
A curve g1 and a curve g2 indicate the relations when observing along the 0° azimuth and 180° azimuth at a pretilt angle of 88°.
A curve h1 and a curve h2 indicate the relations when observing along the 0° azimuth and 180° azimuth at a pretilt angle of 85°.
Referring to each pair of the curve e1 and curve e2, curve f1 and curve f2, curve g1 and curve g2, curve h1 and curve h2, description will be made on comparison between these pairs. The Re dependencies of the optical transmittance are quite the same when observing along the 0° azimuth and 180° azimuth at the pretilt angle of 90°. However, the Re dependency of the optical transmittance when observing along the 0° azimuth at a pretilt angle other than 90° is different from that when observing along the 180° azimuth. There is a tendency that a difference between optical transmittances along both the azimuths at the same Re becomes larger as the pretilt angle becomes smaller.
One curve of each of the four pairs of the curves crosses the other curve at Re of near 50 nm. At this cross point, LCD can be observed at the equal optical transmittance along both the 0° and 180° azimuths.
As seen from the cross points between the curve e1 and curve e2, between the curve f1 and curve f2, between the curve g1 and curve g2, and between the curve h1 and curve h2, as the pretilt angle becomes smaller, the optical transmittance at the cross point (point where the optical transmittances are equal when viewing from the right and left sides) becomes larger. According to the study results by the present inventor, this tendency exhibits also for LCD using a combination of the A and C plates in place of the biaxial film. These simulation results are coincident with external observation of LCD.
Transmittances were simulated for LCD having a C plate as the viewing angle compensation plate between the lower substrate and lower polarizer of the mono-domain pretilt vertical alignment type LCD shown in FIG. 7, along the 0° and 180° azimuths, at a polar angle of 50° while the retardation Δnd of the liquid crystal layer is changed. A thickness direction retardation Rth of the C plate was adjusted to Rth=Δnd−140 nm. The pretilt angles on the upper and lower substrates were fixed to 89°.
The simulation results are shown in FIG. 11. It can be seen that as the retardation Δnd becomes larger, a transmittance at the 0° azimuth increases and a transmittance at the 180° azimuth decreases, and a difference therebetween becomes larger.
It can be seen that at Δnd of 0.58 μm or larger, a transmittance difference between the 0° and 180° azimuths is a twofold or more. Namely, it can be seen that as the retardation Δnd becomes larger, a transmittance difference between the 0° and 180° is definite also from external observation. It is remarkable particularly at Δnd≧0.58 μm. This phenomenon does not appear at all at the pretilt angle of 90°. Conversely, as the pretilt angle becomes smaller, this phenomenon can be observed as more remarkable effects. This phenomenon occurs also when the biaxial film or a combination of the A and C plates is used as the viewing angle compensation plate.
The present inventor has pointed out in JP-A-2004-267160 that a uniform mono-domain alignment cannot be obtained at a pretilt angle of 90° to 89.5° of liquid crystal molecules positioned at the center of the liquid crystal layer in a thickness direction in a mono-domain pretilt vertical alignment type LCD whose substrate surfaces were subject to an alignment process such as shown in FIG. 7, and that it is therefore preferable to set the pretilt angle to 89.5° or smaller in the central area of the liquid crystal layer of a mono-domain pretilt vertical alignment type LCD whose one or both substrates were subject to the alignment process. If the pretilt angle is set to about 90°, there is a possibility that the alignment process method and an alignment film material are limited so that it is not preferable from the viewpoint of narrowing a margin of LCD manufacture.
When considering the indication in JP-A-2004-267160 and the above-described simulation results, it may be considered that it is difficult to manufacture LCD having a high contrast and an optical transmittance symmetry both on the right and left sides.